(a) Color calibration—Color reproduction is affected by printer variables, which can include different drop volumes of different pens, firing energies from the printer system itself, and kinds of media. Manufacturing tolerances, often quite broad—in the interest of economy—in these and other printer parameters result in a deviation between the actual and the targeted color.
To correct such deviations, it is known to include in a printer an automatic color-calibration algorithm, which can compensate for these differences. The idea is to calibrate the printer so that it acts as a nominal printer.
A representative approach to such corrections involves transfer functions (FIG. 1) preferably in the form of one-dimensional lookup tables (LUTs)—one transfer function for each channel (cyan C, magenta M, yellow Y and black K). Each transfer function is intended to perceptually linearize the response of the corresponding channel.
Preparing such transfer functions necessarily begins with some form of comparison between the actual output and the ideal output of a printer, in response to a known input. Such comparison in turn calls for printing a test pattern and then measuring it calorimetrically (or at least “pseudodensitometrically”, as defined in the above-mentioned patent document of Thomas Baker)—in general a well-known procedure, with many variants.
Many incremental printers have a so-called “line sensor” that is mounted with the marking devices (“pens” in an inkjet machine) and typically provided for facilitating mechanical enhancements such as pen alignment. Line sensors have been found adequate for simple densitometric measurements and accordingly in many printers are now commonly put into service for color calibration as well. The Baker document includes extensive orientation to such sensors and their pseudodensitometric use, and related ways of alternatively obtaining more-precise measurements for calibration, e. g. through use of an onboard calorimeter.
(b) Linearity and stability—In all such efforts it is necessary to confront certain sources of measurement inaccuracy and imprecision. Virtually all equipment used for such measurements is subject to displacements of both measurement zero and range; and such measurement displacements must be carefully controlled to avoid collecting meaningless data. (Here the word “range” does not mean distance, but only means a tonal or colorimetric interval between zero and full-scale.)
Displacement of the measurement zero point is commonly and adequately managed by taking a reading without illumination. Range displacement most typically is more difficult to bring under control because it implicates the linearity and stability of every element—optical, electronic and mechanical—in the signal train.
Linearity is a requirement for sensitivity and accuracy (absolute correctness of what is measured). Stability is a requirement for precision (reproducibility)—and for accuracy too, as numbers can hardly be accurate if they are uncertain.
In particular the optical part of the signal path begins with illumination, i. e. the light source. This part of the path also extends, through the visual/mechanical properties of the printing medium, to the detector and the optoelectronic conversion which it performs.
The electronic part of the signal path begins with electronics that drive the light source, and picks up again where the detector hands off an analog electrical signal to (modernly) an analog-to-digital converter (ADC). Even though the illumination end of the train requires only a single stable level, drift in the source excitation or its electrooptical conversion efficiency is a severe limitation—but usually accommodated satisfactorily by allowing time for the source to warm up completely before measurements begin.
At the other end of the optical train, the detector and ADC too are subject to drifts but these are normally under control when the source drifts are. The detector and its ADC nevertheless have the far greater challenge of responding not just at a single level but linearly over the full possible range of the optical signals from the test pattern.
Linearity here is essential, since the whole point of the calibration—as mentioned above—is to develop a compensation for perceptual nonlinearity in the printing system. This requires that the detector and ADC be capable of sensitively discriminating and quantifying very small differences in signal level, and these small differences are necessarily measured superimposed upon some fairly substantial absolute level.
In other words, the measurement system must be able to quantify small differences between big numbers, and this ordinarily calls for high-quality, very sensitive and linear equipment. The computer-printer field, however, is extraordinarily price-competitive—and a detector and ADC of such quality are costly.
(c) Secondary standard—A known approach to mitigating the demands on these measurement elements is to provide a reference measurement at or near the full-scale level of the measuring system, so that at least the range itself is well defined. Thus it is commonly known to expose the detector to the printing medium, in a region that has no printing—i. e. that is bare printing medium—shortly before using the detector to make color measurements of a test pattern.
When this is done, the sensor system need not be itself a good absolute standard but only a secondary standard, since it is referred to the optical reference. Again, this approach mitigates, but does not eliminate, demands on the measurement elements.
This strategy still depends upon linearity in the detector and ADC. These devices are ordinarily adequate in linearity, provided that the d. c. level (the pedestal on which the small signal differences are superposed) is not excessively high; and provided also that the warmup period mentioned above is sufficient for good stability in the lamps, detector and ADC.
(d) Linearity and cost—In one concurrently developed system (not prior art), it has been found that these requirements can be met if the ADC is a relatively expensive unit. One key reason for this expense is that the d. c. signal pedestal is in fact quite elevated, placing stringent demands on sensitivity of the ADC, and this reason in turn arises—interestingly enough—directly from spectral requirements on the light source, as will now be explained.
In these systems it is necessary to use lamps that provide good illumination throughout the visible spectrum, since the inks in use necessarily span all those colors. Favored sources nowadays are light-emitting diodes (LEDs), and it is necessary to use two such devices to cover the visible colors. An LED, like most lamps, is notoriously temperature dependent in emission intensity, spectral distribution, and in some cases even spatial distribution of intensity across the beam.
In practice to achieve sufficient warmup for the needed stability, both LEDs must be turned on continuously throughout the measurement of the entire test pattern. Therefore even when the detector and ADC are measuring linearity for a particular ink that requires light from only one lamp, the other lamp is running too.
Thus the sensor and ADC are forced into use in an unfavorable mode: the small differences are as small as always, but the “large number” (the d. c. level) is generally doubled. Although this difficulty has been couched in terms of the LEDs currently favored, a similar unfavorable relationship would arise even for a single, spectrally continuous source.
As noted above, this relationship has been found acceptable if an adequately linear ADC is in use, but this condition requires a relatively costly ADC. In the concurrently developed system mentioned above, the ADC is a very sensitive twelve-bit unit. Such cost can be made acceptable in a high-end system, but in a more economical printer is very undesirable.
Thus the problem to be solved is how to provide signal linearity and sensitivity or “discrimination”, adequate for a good color calibration—more economically. An inexpensive ADC would suffice if the required linearity were not so high—in particular, if the signal pedestal could be roughly halved—and this would be the case if the light level were lower.
Full spectral illumination with adequate warmup, however, call for a high light level as described above. Accordingly a solution to the stated problem heretofore has been elusive.
(e) Sensor proximity—Surprisingly, even the above-mentioned concurrently developed high-end system, notwithstanding its relatively higher cost, has been found to exhibit a peculiar kind of instability in the calibration process. In this system as already noted the ADC is quite sufficiently linear to enable full preliminary warmup of light sources and operation with optical signals that are superposed on a large pedestal as outlined in the preceding subsection.
Nevertheless, despite amply adequate sensor stability per se, accurate calibration has been found elusive in this system. Upon careful analysis, what at first seemed to be an entirely erratic measurement offset—varying during the course of the calibration measurements—was traced to a systematic variation with the tonal level being measured.
Although systematically related to tonal level, the variation was not proportional to the tonal level but nevertheless was correlated with particular tonal values. In due course it was discovered that the correlation actually was with position of the sensor-holding carriage along its scan path—and, finally, with variations (“runout”) in the distance of the sensor from the printing medium, during the scanning motion.
In other words, the sensor-to-medium distance varies systematically along the scan axis. In retrospect this is not entirely surprising, since the scan path over the printing medium is nearly 1½ (five feet) long and the sensor runout quite tiny.
Nevertheless these minuscule displacements are more than sufficient to cause major fluctuations in light reflected from the printing medium to the detector. Particularly awkward is a distinct nonlinearity of these displacements, and of the resulting optical fluctuations, with position along the scan axis. What is called for is some means of compensating for this curious source of calibration error.
(f) Conclusion—As this discussion shows, limitations of linearity, stability and price in economical systems—and also of mechanical tolerances in even a relatively expensive system—continue to impede achievement of uniformly excellent color calibration and therefore inkjet printing. Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.